There are a number of medical conditions for which it has been found that an effective therapy involves driving current through a section of the tissue of a patient. Often, the current is driven between the electrodes of an electrode array implanted in the patient. Generally, the electrode array includes a non-conductive frame on which typically two or more electrodes are disposed. Once the electrode array is implanted, current is driven from at least one of the electrodes, through the adjacent tissue, to at least one of the other electrodes. The current flow through the tissue influences the tissue to accomplish a desired therapeutic result. For example, an electrode array positioned adjacent the heart may flow currents to stimulate the appropriate contraction and expansion of the heart muscles. There is an increasing interest in implanting electrode arrays adjacent neurological tissue so that the resultant current flow induces a desired neurological or physical effect. In one known application, the current driven between the electrodes of such an array reduces the extent to which chronic pain signals are sent to the brain. This type of therapy is sometimes referred to as neuromodulation for pain management. Alternatively, the current flow stimulates a feeling of stomach fullness as part of an appetite suppression/weight management therapy. In another application, the current is flowed to tissue or nerves associated with the bladder or the anal sphincter to assist in control of incontinence. Electrodes may be implanted in a paralysis victim to provide muscle control and/or a sense of feeling.
Often, the current driven between the electrodes is sourced from a pulse generator also implanted in the patient, known as an implantable pulse generator (IPG).
Once the electrode array of the above assembly is implanted, current can be driven between selective sets of electrodes. By convention, one electrode is often referred to as the anode and the complementary electrode the cathode. In practice, since the current is typically a biphasic current; at one instant during the operation of the electrode array a particular electrode may function as an anode and, in the next instant a cathode.
The term “cathode” is typically associated with the electrode that, during the first phase of a biphasic pulse serves as a current source. The term “anode” is typically associated with the electrode that, during the first phase of a biphasic pulse serves as a current sink.
There are a number of different protocols for driving current between the electrodes of an array. One protocol is monopolar stimulation. During monopolar stimulation, current is driven from/to one or a plurality of electrodes on an electrode array to/from the case of the implanted pulse generator. The IPG thus functions as the complementary electrode to/from which the current is driven. Given the large surface area of the case, and it's relatively large distance away from the implanted electrode/electrodes, the current flowing through the tissue adjacent the case is very weak. Consequently, this current essentially has no effect on this tissue.
Alternatively, the array may be operated in a bipolar mode. In the bipolar mode, two of the electrodes on the array serve as the complementary electrodes between which the current is driven. When current is driven between three electrodes, the array is considered to be operating in a tripolar mode. Typically, when current is driven in the tripolar mode, there is a center electrode and two outer electrodes. At any given instant, the charge of the two outer electrodes is of a first polarity while the charge of the center electrode is of a second, opposite polarity.
When an array is operating in either the bipolar or tripolar mode, the current flow between the electrodes is, in comparison to monopolar stimulation, more focused. This allows more selective targeting of the tissue through which the current is to be flowed.
In bipolar or tripolar systems, the case of the IPG often functions as a neutral or reference electrode. Cathodal and anodal pulses are, respectively of negative and positive potential relative to the IPG case.
It is further understood that, regardless of whether the array is operated in the monopolar, bipolar or tripolar mode the array is operated so that charge balancing occurs. This means that the electrodes are operated so that, at any given instant, the current sourced by some of the electrodes is the same current that is sunk by the complementary electrodes. This charge balancing substantially eliminates the flow of leakage current through tissue away from the target tissue to the IPG case. Charge balancing therefore reduces the potentially adverse effects of such leakage current.
Further, it is known to operate the electrode array assembly so that the charge around each electrode is balanced on a pulse-by-pulse basis. This charge balancing around each electrode is performed to prevent a net direct current from being applied to the tissue adjacent the array and the electrodes themselves. This direct current has been known to damage both electrodes as well as the tissue adjacent the electrodes.
This balance can be performed either passively or actively. Passive balancing is achieved by placing a capacitor in series with the electrode to store/draw residual polarized charge local to the tissue-electrode interface away from the tissue. The capacitor effectively prevents the passage of a net direct current. However, to ensure this balancing, the capacitor must be given sufficient time to discharge. During this discharge period, there may be sufficient current flow that either the electrode or underlying tissue could be damaged.
Active charging balancing is achieved by applying biphasic stimulation pulses to an electrode. In this method, the charges sourced during the opposed phases of the pulse must be equal to each other to achieve total charge balance on the face of the electrode.
In the active charge balancing processes, the initial phase of the biphasic stimulation process is referred to as the lead phase. The second phase is referred to as the balancing phase. Often the magnitude of the current flow during the balancing phase is less than that of current flow during the lead phase. Sometimes, the magnitude of the current flow during the balancing phase is so low, it does not trigger a response in the tissue through which the current is flowed. The current flowed during the balancing phase thus serves only to achieve the desired individual electrode charge balancing.
Many electrode arrays have more than three electrodes. Once an array is implanted, the current is initially driven between different sets of electrodes. The goal of this experimentation is to find the current path through the tissue that results in the most benefit and/or least adverse side effects to the patient. This can result in the array operating in a state in which one or more electrodes neither serve as current sources or current sinks.
Many electrode assemblies used in pain management therapy procedures and other therapies are shaped like flexible, elongated rods. This type of electrode assembly has a diameter typically no greater than 2 mm. At least for pain management applications, the electrode assembly is so sized so it can be positioned in the epidural space within a vertebral column, the space between the ligamentum flavum and the spinal cord dura. More particularly the electrode assembly is positioned in this space on or near the spinal cord dura. The electrode assembly is sufficiently miniaturized so it can be introduced through a cannula or needle-like delivery device. This eliminates the need to invasively cut through the paraspinal muscles, interspinus ligament, ligamentum flavum and expose portions of the lamina to gain access to the epidural space to position the electrode. The electrode assembly itself includes a number of longitudinally spaced apart electrodes. Once the electrode assembly is positioned adjacent the dura, current pulses are applied between selected sets of electrodes. These current pulses flow, in part, through the spinal cord. The electrode current flow patterns are experimented with until the individual reports, instead of pain, a more tolerable tingling sensation. This tingling sensation is known as paresthesia.
The above therapy offers some relief to many individuals suffering from chronic pain. One disadvantage of these assemblies is that their construction provides a relatively low spatial resolution of spinal cord stimulation. If the current pulses cannot be directed through the sections of the dorsal column that are most closely associated with the neurons through which the pain signals are being transmitted, the application of the signal may not result in appreciable masking of the pain signals. One of the few ways to compensate for this imprecise targeting is to over stimulate the dorsal column nerves. This may result in some nerves being subjected to needless stimulation. Moreover, the potential fields generated between individual electrodes tend to drive currents radially. The emission of a fraction of this current away from the spinal cord is needless expenditure of electrical energy. This can be a significant drawback in an implanted device in which the available power is limited. Also, owing to the rounded and flexible shape of this type of electrode assembly, it can shift position along the spinal cord. Should this event occur, the current pulses directed between the individual electrodes may no longer be of any use for inhibiting the transmission of pain signals.
There have been attempts to overcome some of the above limitations by providing implantable electrode systems that include electrode arrays that are paddle-shaped. The electrodes integral with a paddle shaped electrode array are typically spaced closer together than the electrodes of a rod type electrode array assembly. Further, the electrodes of a paddle-type electrode array are typically positioned towards the surface of the dura. To implant this type of assembly, it is necessary to invasively cut through the para-spinal muscles, interspinus ligament, ligamentum flavum and portions of the lamina. This implantation requires a relatively invasive surgical procedure.